A sintered metal bearing is used suitably as an oil-impregnated sintered bearing obtained, for example, through impregnation of a lubricating oil and the like into inner pores; specifically, used at parts required to have excellent bearing performance and durability, such as bearing components for automobiles and a motor spindle for information apparatuses.
Here, the sintered metal bearing is used, for example, in a fluid dynamic bearing device. The fluid dynamic bearing device is a bearing device which rotationally supports a shaft member in a non-contact manner with use of a dynamic pressure action, which is generated in a bearing gap, of a lubricating oil or the like. The bearing device has characteristics such as high-speed rotation operation, high-speed rotational accuracy, and quietness. In recent years, by taking advantage of those characteristics, the bearing device has been suitably used as a bearing device for a motor to be mounted to various electrical apparatuses such as information apparatuses. Specifically, as a bearing device for a motor, the bearing device has been suitably used in the following: a spindle motor for a magnetic disk drive such as an HDD, an optical disk drive for a CD-ROM, a CD-R/RW, a DVD-ROM/RAM, or the like, or a magneto-optical disk drive for an MD, an MO, and the like; a polygon scanner motor for a laser beam printer (LBP); or a fan motor.
The above-mentioned various motors are required to have higher high-speed rotational accuracy. Bearings for supporting a spindle for the motors are one of factors which determine the required performance. In recent years, there has been studied a use of fluid bearings which have such an excellent property as to meet the required performance, or the fluid bearings have been actually used.
Fluid bearings of this type are roughly classified into dynamic pressure bearings provided with a dynamic pressure generating portion for generating dynamic pressure in a lubricating fluid in a bearing gap, and so-called perfectly cylindrical bearings (bearings whose sectional configuration is perfectly circular) provided with no such dynamic pressure generating portion.
For example, in a fluid dynamic bearing device incorporated in a spindle motor for a disk drive device, such as an HDD, both the radial bearing portion supporting the shaft member in the radial direction and the thrust bearing portion supporting the shaft member in the thrust direction may be formed by dynamic pressure bearings. As the radial bearing portion in the fluid dynamic bearing device of this type (dynamic bearing device), there has been conventionally well-known one which forms the following: as dynamic pressure generating portions, regions, in which a plurality of dynamic pressure generating grooves are arranged, for example, on a radially inner surface of a sleeve formed of a sintered metal; and radial bearing gaps between the radially inner surface on which the dynamic pressure generating portions are formed and a radially outer surface of the shaft member, the radially outer surface facing the radially inner surface (for example, refer to Patent Literature 1 below).
As described above, the sintered metal bearing for many uses is formed, for example, as disclosed in Patent Literature 2 below, through impregnation of a fluid such as a lubricating oil or a lubricating grease into a porous body obtained by sintering of a Cu powder, an Fe powder, or a metal powder containing both of those powders as main components, the powders having been compression-molded into a predetermined shape (cylindrical shape in many cases).
Here, Patent Literature 3 below discloses an oil-impregnated sintered bearing obtained by compression-molding and sintering subsequent thereto of a mixed metal powder containing a Cu powder and an SUS powder.
Alternatively, Patent Literature 4 below discloses an example of a sintered metal bearing containing an Fe-based powder as a main component. Specifically, there has been proposed an Fe-based sintered sliding member formed of a Cu-based alloy powder, a Cu powder, a carbon powder, and an Fe power. The Fe-based sintered sliding member contains the powders at the following weight ratios: a Cu component of 15 to 25 wt %; an Si component of 1 to 5 wt %; an Sn component of 1 to 5 wt %; a carbon component of 3 to 10 wt %; and a balance Fe component (55 to 80 wt %). Further, Patent Literature 4 discloses that the sintered sliding member is manufactured by powder-press molding of a mixed powder obtained through such formulation as to achieve the above-mentioned weight ratios, and then by sintering at 1,100 to 1,150° C. of a powder-press body thus obtained.
Further, a bearing sleeve formed of a sintered metal (sintered metal bearing), which is used by being incorporated in the above-mentioned fluid dynamic bearing device, is similarly formed by sizing of a sintered raw material, that is, a sintered compression-molded body obtained by compression-molding of a metal powder containing a Cu powder, an Fe powder, or both of those powders into a predetermined shape with use of a die (for example, refer to Patent Literatures 5 and 6 below).
FIGS. 29 to 31 illustrate a schematic structure of a sizing apparatus as an example. This apparatus includes, as main components, a cylindrical die 313 into which a radially outer surface 311b of a sintered raw material 311 is press-fitted, a sizing pin 312 for molding a radially inner surface 311a of the sintered raw material 311, and an upper punch 314 and a lower punch 315 for pressing both end surfaces of the sintered raw material 311 from an upper-and-lower direction. On an outer peripheral surface of the sizing pin 312, there are provided a projection-recess molding die in conformity with a shape of a bearing surface of a finished product. Projecting portions of the molding die form regions of dynamic pressure generating grooves in a bearing surface, and recessed portions forms regions other than the dynamic pressure generating grooves.
In sizing, first, as illustrated in FIG. 29, the sintered raw material 311 is arranged in such a manner as to be positioned to an upper surface of the die 313. In this case, a predetermined press-fitting margin D301 is secured between the radially outer surface 311b of the sintered raw material 311 and a radially inner surface of the die 313 into which the sintered raw material 311 is to be press-fitted, and a radially inner gap D302 exists between the radially inner surface 311a of the sintered raw material 311 and the molding die (projecting portions) of the sizing pin 312 under a state prior to press-fitting into the die 313.
After that, the upper punch 314 and the sizing pin 312 are lowered so that the sintered raw material 311 is press-fitted into the die 313. As illustrated in FIG. 30, the upper punch 314 is pressed-in up to a bottom dead center, and the sintered raw material 311 is pressed onto an upper surface of the lower punch 315 so as to be pressurized from the upper-and-lower direction. The sintered raw material 311 deforms by receiving a compressive force from the die 313, the upper punch 314, and the lower punch 315, and the radially inner surface 311a is pressurized in conformity with the molding die of the sizing pin 312. A pressurizing amount of the radially inner surface 311a is substantially equal to a difference between the press-fitting margin D301 and the radially inner gap D302. A surface layer part corresponding to a predetermined depth from the radially inner surface 311a is pressurized in conformity with the molding die of the sizing pin 312, and is plastically fluidized so as to adhere to the molding die. With this, the shapes of the molding die are transferred to the radially inner surface 311a of the sintered raw material 311, and the bearing surface is molded (simultaneously, the radially outer surface 311b of the sintered raw material 311 undergoes sizing). In this case, until the upper punch 314 reaches the bottom dead center so as to compress the sintered raw material 311 after the upper punch 314 is inserted into the die 313, the lower punch 315 stands by in the die 313 and maintains its position.
After that, as illustrated in FIG. 31, the sizing pin 312, the upper punch 314, and the lower punch 315 are raised so as to pull out the sintered raw material 311 from the die 313, with a positional relation between the molding die and the sintered raw material 311 being maintained. When the sintered raw material 311 is pulled out from the die 313, springback occurs in the sintered raw material 311, and an inner diameter dimension increases. With this, the sintered raw material can be pulled out from the sizing pin without damage to the shapes of the dynamic pressure generating portions.